Currently, x-rays present a wide range of applications in the fields of nondestructive detection, security check and medical diagnoses and treatment etc. High-energy x-rays are required for the fluoroscopy inspection of large detection objects, e.g. boilers, aerospace engines, bulk cargos in the airport or at the customs or transported by the railway. In such cases, high-energy x-rays are generated by the electron accelerator with the energy above 2 MeV.
Generally, the basic method that the electron accelerator generates x-rays includes: generating electron beam current by an electron gun; providing the electron beam current with high energy via the acceleration by the electric field; and generating x-rays as the high-energy electron beam current bombards the target. The area where x-rays (or useful x-rays) are distributed is termed as x-rays radiation field. The x-rays generated via the bombarding by the electron beam current are distributed divergently to each orientation in 4Π solid angle. The intensity distribution of the x-rays generated via bombarding the target by the electron beam current with different energy in each transmitting direction is various. Generally, the higher the energy of the electron beam current is, the larger the intensity of the forward x-rays is. In general, the moving direction of the electron beam current is defined as forward. With regards to the intensity of the x-rays generated via bombarding the target by the high-energy electron beam current in various directions, the intensity of the x-rays generated via bombarding the target by the high-energy electron beam current in the forward direction is the biggest, and decreases as the angle deviates the forward direction increases. The higher the energy of the electron beam current is, the more obvious the change is. For example, take the x-rays generated via the bombarding by the electron beam current with the energy of 9 MeV (million electron volts) as an example, if the intensity of the x-rays in the center (forward) is 1, the intensity of the x-rays in the direction deviating the center 5 degrees is about 73%, 10 degrees about 53%, 15 degrees about 40%, 30 degrees about 18%. This is a very obvious forward concentrating distribution. In the inspection system which performs fluoroscopic imaging by the x-rays generated by the electron accelerator, the larger the volume of the objects to be inspected is, the higher the energy of the required x-rays is, and the distributed angle of the x-rays required is larger. However, the intensity distribution of the x-rays radiation field with high energy and large distributed angle is extremely nonuniform which decreases the quality of the detected image dramatically. In addition, in the aspect of medical radiation therapy, because the intensity distribution of the x-rays radiation field is nonuniform, the severe problem that the central (forward) area is radiated excessively whereas the marginal area is radiated insufficiently is occurred.
In the prior art, to eliminate the adverse effect imposed on the image quality caused by the non-uniform intensity distribution of the radiation field generated by the x-rays of forward concentration, or to eliminate the non-uniform radiation in the radiotherapy, in general, some blocking devices, termed as flattening filter, are arranged in front of the target generating the x-rays, such that the intensity of the forward x-rays within the small range weakens rendering the intensity distribution of the x-rays within a certain range of angles relative uniform. This way of doing is “Shaving high and approaching low” which sacrifices the maximum intensity of the radiation field generated by the electron accelerator resulting in the reduced utilization efficiency, the blurry target spots in a radiation fluoroscopic imaging system and a lowered image resolution.